Why is my battery level draining so fast but then goes back up without charging?

Background Info:
I'm a college student with a Samsung Galaxy Tab 3 7.0 and I've had it for a little more than a year. I haven't had many issues with it as I mostly use it for light tasks like social media and note-taking. On trips I'll use it for movies and music, but again nothing strenuous. I'll be the first to admit that I'm guilty of letting it charge all night, every night even when it may not need it.

The Issue:
The past few weeks, my battery level has been fluctuating big time. It will say 100% and then it says 1% in the matter of an hour. Then the battery level will go back up on it's own, without charging, sometimes as high as 95% but then it will drop back down. It will also go down to 1% and stay there for hours without shutting down. This can get frustrating when I'm trying to take notes in class, but can't because I can't see the screen as the brightness goes all the way down by default.

At first I thought it was a battery issue, but then noticed that I would stay on 1% for hours. It went almost two days with light usage, before it would shut down. So this seems like it may be a software issue rather than a hardware issues. I tried doing a factory reset, but there was no change. Is there anyone else having this issue? Is there a fix?

Welcome to Android Central! You could try recalibrating the battery meter. Charge up to full, then use the tablet until it shuts off due to low battery. Repeat the process 3 or 4 times. Then see if the behavior persists.

Sounds more like a dendrite. Lithium Battery Dendrite links has more than you want to know about them - except that the only "fix" is to replace the battery.

BTW, if your phone is rooted, Battery Calibration (Root) will make calibrating it easier. (Charge until the battery stops charging [not necessarily when it says 100%], then tap a button and it's done.)